Quick HOWTO : Ch03 : Linux Networking/zh
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介绍
现在你已经牢牢掌握了许多最常用的网络概念，是时候应用它们来配置你的服务器了。其中一些设置在Linux安装过程中已经自动配置过了，但是你经常会发现你必须知道怎样修改这些初始设定。无论是你需要移动你的服务器到另一个网络中，还是添加一块新的网卡或者使用另一种方式连接到Internet。
在第二章中， "Introduction to NetworkingCh02 : Introduction to Networking"，我们首先解释了什么是TCP/IP，所以我们也将从讨论怎样配置服务器的IP开始Linux的网络这一章。
怎样配置网卡的IP地址
你需要知道的一切必要步骤来配置网卡的IP地址。网站的购物车应用可能需要额外的固定IP地址。你也可能需要增加第二块网卡，用于处理服务器的数据备份。最後，但并不少见，你可能只想在服务器上玩玩以测试你的技巧。
本节将展示怎样处理最常见的服务器IP操作。
确定你的IP地址
大多数现代电脑都有一个以太网端口。在Linux安装中，这个设备被称为eth0。你可以通过ifconfig命令确定这个设备的IP地址。
[root@bigboy tmp]# ifconfig -a eth0 Link encap:Ethernet HWaddr 00:08:C7:10:74:A8 BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:0 (0.0 b) TX bytes:0 (0.0 b) Interrupt:11 Base address:0x1820 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:787 errors:0 dropped:0 overruns:0 frame:0 TX packets:787 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:82644 (80.7 Kb) TX bytes:82644 (80.7 Kb) wlan0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5 inet addr:192.168.1.100 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:47379 errors:0 dropped:0 overruns:0 frame:0 TX packets:107900 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:4676853 (4.4 Mb) TX bytes:43209032 (41.2 Mb) Interrupt:11 Memory:c887a000-c887b000 wlan0:0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5 inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:11 Memory:c887a000-c887b000 [root@bigboy tmp]#
在这个例子中，eth0是没有IP地址的，因为这台机器用无线网卡wlan0作为主网卡。接口wlan0有一个IP地址192.168.1.100和子网掩码255.255.255.0。
你可以看到，这个命令提供了每块网卡的中断或者PCI总线ID等有用信息。在某些罕见情况中，你可能发现你的网卡不工作是因为它与其他一些设备共享了中断和内存访问地址。你可以通过查看/proc/interrupts文件来获得系统使用的全部中断请求（IRQs）。在下面的例子中， 我们可以看到从IRQ 0 到 15没有中断冲突，每个中断只有一条记录。设备eth0和eth1分别使用了中断10和5：
[root@bigboy tmp]# cat /proc/interrupts CPU0 0: 2707402473 XT-PIC timer 1: 67 XT-PIC i8042 2: 0 XT-PIC cascade 5: 411342 XT-PIC eth1 8: 1 XT-PIC rtc 10: 1898752 XT-PIC eth0 11: 0 XT-PIC uhci_hcd 12: 58 XT-PIC i8042 14: 5075806 XT-PIC ide0 15: 506 XT-PIC ide1 NMI: 0 ERR: 43 [root@bigboy tmp]#
如果有冲突，您可能需要参考引发问题设备的手册，以确定是否可以用另一个中断或内存I/O地址。
改变你的IP地址
如果需要，你可以使用ifconfig命令为eth0指定一个IP地址。
[root@bigboy tmp]# ifconfig eth0 10.0.0.1 netmask 255.255.255.0 up
命令行结尾的"up"会激活这个设备。要让这个设定在你每次启动时都生效，你需要把这个命令加到/etc/rc.local文件中。它会在每次启动时执行。
Fedora Linux通过在/etc/sysconfig/network-scripts目录下的配置文件，使这个设定变得简单。设备eth0对应文件ifcfg-eth0，eth1对应ifcfg-eth1，等等。你可以把IP地址信息放到这些文件中，它们会在Linux启动时自动配置你的网卡。见Figure 3-1两个eth0设备的例子。一个假设该网卡有一个固定IP地址，另一个假设它需要通过DHCP获得IP地址。
图 3-1 - network-scripts的文件格式
| 固定IP地址 |
[root@bigboy tmp]# cd /etc/sysconfig/network-scripts [root@bigboy network-scripts]# cat ifcfg-eth0 # # File: ifcfg-eth0 # DEVICE=eth0 IPADDR=192.168.1.100 NETMASK=255.255.255.0 BOOTPROTO=static ONBOOT=yes # # The following settings are optional # BROADCAST=192.168.1.255 NETWORK=192.168.1.0 [root@bigboy network-scripts]# |
| 通过DHCP获得IP地址 |
[root@bigboy tmp]# cd /etc/sysconfig/network-scripts [root@bigboy network-scripts]# cat ifcfg-eth0 # # File: ifcfg-eth0 # DEVICE=eth0 BOOTPROTO=dhcp ONBOOT=yes [root@bigboy network-scripts]# |
你可以看到eth0将在启动时被激活，因为参数ONBOOT的值是yes不是no。你可以在第2章中"Introduction to Networking"阅读更多关于netmask和DHCP内容。
缺省的RedHat/Fedora安装会在network-scripts文件包含广播和网络设定。这些都是可选的。
在改变网卡的配置文件后，你需要停止然后激活它以使修改生效。ifdown和ifup命令可用于实现这个目的：
[root@bigboy network-scripts]# ifdown eth0 [root@bigboy network-scripts]# ifup eth0
你的服务器必须有一个默认网关，使它能够与Internet通信。这将在本章的稍后涉及。
DHCP如何影响你使用的DNS服务器
DHCP服务器不仅为你的Linux机器提供IP地址，也包括所需的DNS服务器。当为一个网络设备设置DHCP时，确保在/etc/resolv.conf文件注释掉相关的服务器配置，以防止任何冲突。
为一个网卡设定多个IP地址
在上一节“确定你的IP地址中”大家可能注意到有两个无线网络设备：wlan0和wlan0:0。设备wlan0：0实际上是wlan0的子设备，一个虚拟的子设备也被称为IP别名。IP别名是为一个网卡创建多个IP地址的最常见方法。别名的命名方式是父设备名称:X，X是你选择的子设备号。
创建一个IP别名的过程与上一节“改变你的IP地址”中描述的非常相似：
- 首先，确保真实的父网络设备存在。
- 确认你计划使用的名字没有同名的其他IP别名存在。这里我们要创建一个网络设备wlan0:0。
- 用ifconfig命令创建虚拟网络设备
[root@bigboy tmp]# ifconfig wlan0:0 192.168.1.99 netmask 255.255.255.0 up
- 你也应该一个文件/etc/sysconfig/network-scripts/ifcfg-wlan0:0，这样这个别名就可以被ifup和ifdown命令自动管理。这个是一个配置样例:
DEVICE=wlan0:0 ONBOOT=yes BOOTPROTO=static IPADDR=192.168.1.99 NETMASK=255.255.255.0
下面命令用于激活和停止这个别名设备:
[root@bigboy tmp]# ifup wlan0:0 [root@bigboy tmp]# ifdown wlan0:0
注意: 关闭主网络设备也同时关闭其所有别名。而别名可以独立于其他网络设备关闭。
完成这4个简单的步骤後，你应该能从网络的其他服务器上ping通这个新的IP别名。
为DSL连接设定IP地址
如果你使用是有固定或静态IP地址的DSL连接，那么配置步骤和之前描述的是一样的。你把以太网接口连接到DSL modem上，配置ISP提供的
IP地址，子网掩码，广播地址，和网关信息。重启网络设备後你就应该连上网了。记住，你可能还需要正确地配置DNS服务器。
如果你使用的是有DHCP或或动态IP地址的DSL连接，那么过程就不一样了。你的ISP会为您提供一个PPPoE（PPP authentication over
Ethernet）的用户名和密码，这将使您的电脑在每次启动时透明地登录到互联网上。Fedora的Linux需要安装rp-PPPoE的RPM软件包来支持这
个功能。
注意：除非你特别申请静态IP地址，你的ISP会向你提供一个基于DHCP的连接。而DHCP分配给你的机器或者互联网路由的IP地址可能很多天
都不会改变，你可能会误为是静态的。
下载和安装RPM包并不难。如果你需要复习，第6章"Installing Linux Software"
中涵盖了关于RPM操作的详细内容。 当寻找该文件时，记得PPPoE的RPM文件名通常是以rp-pppoe开头，后面是版本号，例如：rp-pppoe-
3.5-8.i386.rpm。
在安装RPM后，你需要经过若干步骤来完成连接。PPPoE配置将创建立一个名为ppp0的虚拟接口，它将通过物理网络接口eth0的来连接。下面
是你需要做的：
- 备份你的ifcfg-eth0文件。
[root@bigboy tmp]# [root@bigboy tmp]# cd /etc/sysconfig/network-scripts/ [root@bigboy network-scripts]# ls ifcfg-eth0 ifcfg-eth0 [root@bigboy network-scripts]# cp ifcfg-eth0 DISABLED.ifcfg-eth0
- 编辑你的ifcfg-eth0文件，删除IP信息，并且设定在启动时激活。
DEVICE=eth0 ONBOOT=no
- 关闭你的eth0设备。
[root@bigboy network-scripts]# ifdown eth0 [root@bigboy network-scripts]#
- 运行adsl-setup配置脚本
[root@bigboy network-scripts]# adsl-setup
- 它会提示你输入你的ISP用户名，要使用的网络设备（eth0）和是否你需要连接长期保持。如果可能我们都使用缺省设置。
Welcome to the ADSL client setup.� First, I will run some checks on your system to make sure the PPPoE client is installed properly... LOGIN NAME Enter your Login Name (default root): bigboy-login@isp INTERFACE Enter the Ethernet interface connected to the ADSL modem For Solaris, this is likely to be something like /dev/hme0. For Linux, it will be ethX, where 'X' is a number. (default eth0): Do you want the link to come up on demand, or stay up continuously? If you want it to come up on demand, enter the idle time in seconds after which the link should be dropped.� If you want the link to stay up permanently, enter 'no' (two letters, lower-case.) NOTE: Demand-activated links do not interact well with dynamic IP addresses. You might have some problems with demand-activated links. Enter the demand value (default no):
- 接着会提示你输入你的DNS服务器信息。这步会编辑你的/etc/resolv.conf文件。如果你在服务器上以缓存DNS模式运行BIND，就不用输入
直接回车。如果你希望你的ISP的DNS服务器自动提供IP地址，就输入单词server。
- Please refer to Chapter 18, "Configuring DNS", for more information on BIND and
DNS.
DNS Please enter the IP address of your ISP's primary DNS server. If your ISP claims that 'the server will provide dynamic DNS addresses', enter 'server' (all lower-case) here. If you just press enter, I will assume you know what you are doing and not modify your DNS setup. Enter the DNS information here:
- 脚本会提示你输入你的ISP密码
PASSWORD Please enter your Password: Please re-enter your Password:
- 然后它会询问是否后允许常规用户（不是超级用户root）激活/停止这个新的ppp0设备。这可能是需要的，如果你家里有非root的成员需要
访问Internet的话：
USERCTRL Please enter 'yes' (two letters, lower-case.) if you want to allow normal user to start or stop DSL connection (default
yes):
- rp-pppoe包在/etc/ppp目录下有两个样例ip表防火墙脚本，文件名是firewall-standalone和firewall-masq。它们都非常基本并不涵盖将
你的Linux机器设置成web服务器，DNS服务器或者mail服务器的规则。我都推荐选择NONE，然后使用第14章"[[Quick HOWTO : Ch14 : Linux
Firewalls Using iptables|Linux Firewalls Using iptables]]"中的一个基本脚本样例，或者附录II中的更复杂脚本
FIREWALLING
Please choose the firewall rules to use. Note that these rules are very basic. You are strongly encouraged to use a more
sophisticated firewall setup; however, these will provide basic security. If you are running any servers on your machine,
you must choose 'NONE' and set up firewalling yourself. Otherwise, the firewall rules will deny access to all standard
servers like Web, e-mail, ftp, etc. If you are using SSH, the rules will block outgoing SSH connections which allocate a
privileged source port.
The firewall choices are: 0 - NONE: This script will not set any firewall rules. You are responsible for ensuring the security of your machine. You are STRONGLY recommended to use some kind of firewall rules. 1 - STANDALONE: Appropriate for a basic stand-alone web-surfing workstation 2 - MASQUERADE: Appropriate for a machine acting as an Internet gateway for a LAN Choose a type of firewall (0-2): 0
- 接着你会被询问是否在启动时激活连接。大部分人都会选择yes.
Start this connection at boot time Do you want to start this connection at boot time? Please enter no or yes (default no):yes
- 在退出前，你会获得一个摘要，包括你输入参数，那些配置文件将根据你的选择更新：
** Summary of what you entered **
Ethernet Interface: eth0 User name: bigboy-login@isp Activate-on-demand: No DNS: Do not adjust Firewalling: NONE User Control: yes Accept these settings and adjust configuration files (y/n)? y Adjusting /etc/sysconfig/network-scripts/ifcfg-ppp0 Adjusting /etc/ppp/chap-secrets and /etc/ppp/pap-secrets (But first backing it up to /etc/ppp/chap-secrets.bak) (But first backing it up to /etc/ppp/pap-secrets.bak)
- 在最后，它会告诉你激活/关闭这个新的ppp0设备，以及获得设备状态的命令。
Congratulations, it should be all set up! Type '/sbin/ifup ppp0' to bring up your xDSL link and '/sbin/ifdown ppp0'to bring it down. Type '/sbin/adsl-status /etc/sysconfig/network-scripts/ifcfg-ppp0' to see the link status.
- 注意: 这个例子中推荐使用adsl-status命令后面跟上PPPoE接口配置文件名字。不过这个命令会缺省显示ppp0设备的信息，因此在
大部分系统中没有必要加上ifcfg-ppp0文件名。
在你完成安装配置rp-pppoe后，你应该可以通过DSL的DHCP连接到Internet上。
adsl-setup创建的一些重要文件
有三个adsl-setup脚本创建的文件你可能会感兴趣。第一个是ifcfg-pppo文件，包含了设备链路层的连接参数。
[root@bigboy network-scripts]# more ifcfg-ppp0 USERCTL=yes BOOTPROTO=dialup NAME=DSLppp0 DEVICE=ppp0 TYPE=xDSL ONBOOT=yes PIDFILE=/var/run/pppoe-adsl.pid FIREWALL=NONE PING=. PPPOE_TIMEOUT=20 LCP_FAILURE=3 LCP_INTERVAL=80 CLAMPMSS=1412 CONNECT_POLL=6 CONNECT_TIMEOUT=60 DEFROUTE=yes SYNCHRONOUS=no ETH=eth0 PROVIDER=DSLppp0 USER= bigboy-login@isp PEERDNS=no [root@bigboy network-scripts]#
另外两个内容相同的文件时/etc/ppp/pap-secrets和/etc/ppp/chap-secrets，包含了登录ISP的用户名和密码：
[root@bigboy network-scripts]# more /etc/ppp/pap-secrets # Secrets for authentication using PAP # client server secret IP addresses "bigboy-login@isp" * "password" [root@bigboy network-scripts]#
简单的故障诊断
你可以运行adsl-status命令来检查你的连接状态。在这个例子中，可以看到包已经装好但是设备没有被激活。
[root@bigboy tmp]# adsl-status Note: You have enabled demand-connection; adsl-status may be inaccurate. adsl-status: Link is attached to ppp0, but ppp0 is down [root@bigboy tmp]#
激活后，设备即正常工作。
[root@bigboy tmp]# ifup ppp0 [root@bigboy tmp]# adsl-status adsl-status: Link is up and running on interface ppp0 ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1462 inet ... ... [root@bigboy tmp]#
关于更多的故障诊断信息，你可以访问rp-pppoe的网站Roaring Penguin (www.roaringpenguin.com)。那里有一些如果避免VPN客户端问题
的很好的提示。
为Cable Modem设置IP地址
Cable modems使用DHCP来获得IP地址，你可以据此配置你服务器的以太网卡。
怎样激活/关闭你的网卡
命令ifup和ifdown分别用于激活和停止一块网络设备。你必须有一个相关的ifcfg文件在/etc/sysconfig/network-scripts目录下来确保这些命令工作。下面是一个eth0设备的例子：
[root@bigboy tmp]# ifdown eth0 [root@bigboy tmp]# ifup eth0
怎样查看你当前的路由表
netstat -nr命令会显示路由表的内容。网关为0.0.0.0的网络通常是直连到网络设备上的。因为到自己的直连设备上是不需要网关的，所以0.0.0.0的网关地址是正常的。目的地址是0.0.0.0的路由是你的缺省网关。
- 在这个例子里,有两个网关，缺省的并且是到255.255.255.255通常是DHCP服务器添加的。在这个例子里bigbogy服务器也是一个DHCP服务器。
[root@bigboy tmp]# netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 255.255.255.255 0.0.0.0 255.255.255.255 UH 40 0 0 wlan0 192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 wlan0 127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo 0.0.0.0 192.168.1.1 0.0.0.0 UG 40 0 0 wlan0 [root@bigboy tmp]#
- 在这个例子里，有多个网关负责把数据从不同网络设备上传输到不同的目的地。
[root@bigboy tmp]# netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 172.16.68.64 172.16.69.193 255.255.255.224 UG 40 0 0 eth1 172.16.11.96 172.16.69.193 255.255.255.224 UG 40 0 0 eth1 172.16.68.32 172.16.69.193 255.255.255.224 UG 40 0 0 eth1 172.16.67.0 172.16.67.135 255.255.255.224 UG 40 0 0 eth0 172.16.69.192 0.0.0.0 255.255.255.192 U 40 0 0 eth1 172.16.67.128 0.0.0.0 255.255.255.128 U 40 0 0 eth0 172.160.0 172.16.67.135 255.255.0.0 UG 40 0 0 eth0 172.16.0.0 172.16.67.131 255.240.0.0 UG 40 0 0 eth0 127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo 0.0.0.0 172.16.69.193 0.0.0.0 UG 40 0 0 eth1 [root@bigboy tmp]#
怎样改变你的缺省网关
你的服务器需要有一个单一缺省网关。配置了DHCP的网卡会自动由DHCP服务器分配一个缺省网关，但是配置了静态IP地址的网卡就需要手工配置缺省网关。这可以通过一个简单的命令实现。这个例子中缺省路由使用一个新安装的无线网卡wlan0，而大部分PC都是使用标准的以太网卡eth0。
[root@bigboy tmp]# route add default gw 192.168.1.1 wlan0
在这个例子中，请确保IP地址为192.168.1.1的路由/防火墙和wlan0设备在一个网络中！
完成后，你需要更新你的/etc/sysconfig/network文件来反映这个变化。这个文件会在每次Linux启动时配置你的缺省网关。
NETWORKING=yes HOSTNAME=bigboy GATEWAY=192.168.1.1
注意:在以Debian为基础的系统中，缺省网关是固定定义在文件/etc/network/interfaces中。请参阅后面章节"Debian / Ubuntu的网络配置" 以了解更多详情。
有些人可能不愿意修改特定的网络文件，而只是把route add命令添加到/etc/rc.d/rc.local脚本文件中，这样每次启动时就会自动执行了。
也可以在/etc/sysconfig/network-scripts目录下网卡配置文件中定义缺省网关，但这样是有风险的，如果你有多块网卡可能导致定义多个缺省网关。这样会导致连接问题。如果其中一个缺省网关没有到目的地的路由，所有的包都会丢失。而且防火墙会阻挡带有不规则序号和不明来源的包，这也会妨碍你的数据流。
怎样配置两个网关
一些网络可能拥有多个路由器/防火墙提供连接。这里有一个典型的情况：
- 你有一个路由可以访问Internet，你愿意用它来做你的缺省网关（参看前面的缺省网关样例）
- 有还有另一个路由用来访问你的公司网络，公司网络的IP范围是10.0.0.0到10.255.255.255。让我们假定这个路由的IP地址是
192.168.1.254。
这个例子中的Linux机器是使用wlan0提供Internet连接的。你可能是使用eth0设备，请据此调整你的操作。
有多种方法可以添加这个新的路由。
添加临时的静态路由
route add命令可以用来添加一条新路由到你的服务器，直到下一次重启。这个方法的优势是在所有版本的Linux中都通用，而且在man里
面都有详细的说明。在我们的例子里，10.0.0.0网络前面有-net参数，子网掩码和网关前面也分别有netmask和gw参数。
[root@bigboy tmp]# route add -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.1.254 wlan0
如果你想为一个单独的服务器添加路由，那么应该使用-host参数，而且不设子网掩码值（route命令会自动知道子网掩码是
255.255.255.255）。这里是为机器10.0.0.1添加路由的例子。
[root@bigboy tmp]# route add -host 10.0.0.1 gw 192.168.1.254 wlan0
如果想要这个变化在重启后也能持续，一个通用的方法是把route add命令添加到/etc/rc.d/rc.local文件中，它会始终在
启动过程结束前运行。
添加永久静态路由
在Fedora Linux中，永久静态路由是添加在/etc/sysconfig/network-scripts目录下对应设备文件中的。文件名的格式是
route-网络设备名称，所以wlan0设备对应的文件名是route-wlan0
该文件的格式很直观，第一行是目标网络加上单词via和网关的IP地址。在我们的例子中，要设置一个到通过网关192.168.1.254
到网络10.0.0.0的路由，子网掩码是255.0.0.0（一个头8位都是1的掩码），我们像这样设置/etc/sysconfig/network-
scripts/route-wlan0文件：
# # File /etc/sysconfig/network-scripts/route-wlan0 # 10.0.0.0/8 via 192.168.1.254
注意: 文件名/etc/sysconfig/network-scripts/route-*非常重要。在route-后面添加了错误的设备后缀会导致下
次启动后路由不能被正确添加。而且不会在屏幕上报告错误或者在/var/log/目录下生成任何日志文件。
你可以通过运行/etc/sysconfig/network-scripts/ifup-routes命令加设备名作为参数测试新建的文件。在下面例子里，
我们先检查路由表查看没有到网络10.0.0.0的路由，然后执行ifup-routes命名把路由加上。
[root@bigboy tmp]# netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 wlan0 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 wlan0 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 wlan0 [root@bigboy tmp]# ./ifup-routes wlan0 [root@bigboy tmp]# netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 wlan0 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 wlan0 10.0.0.0 192.168.1.254 255.0.0.0 UG 0 0 0 wlan0 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 wlan0 [root@bigboy tmp]#
注意: 在基于Debian的系统中，永久静态路由是配置在文件/etc/network/interfaces中的。请参阅后面章
节"Debian / Ubuntu网络配置"以了解更多详情。
怎样删除路由
这里介绍了怎样删除之前章节添加的路由。
[root@bigboy tmp]# route del -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.1.254 wlan0
文件 /etc/sysconfig/network-scripts/route-wlan0也需要被更新，这样你重启服务器後路由才不会被重新加回来。删除如下的行：
10.0.0.0/8 via 192.168.1.254
Changing NIC Speed and Duplex
There is no better Linux investment than the purchase of a fully Linux compatible NIC card. Most Linux vendors will have a list of compatible hardware on their Web sites: read this carefully before you start hooking up you machine to the network. If you can't find any of the desired models in your local computer store, then a model in the same family or series should be sufficient. Most cards will work, but only the fully compatible ones will provide you with error-free, consistent throughput.
Linux defaults to automatically negotiating the speed and duplex of it's NIC automatically with that of the switch to which it is attached. Configuring a switch port to auto-negotiate the speed and duplex often isn't sufficient because there are frequently differences in the implementation of the protocol standard.
Typically, NICs with failed negotiation will work, but this is usually accompanied by many collision type errors being seen on the NIC when using the ifconfig -a command and only marginal performance. Don't limit your troubleshooting of these types of errors to just failed negotiation; the problem could also be due to a bad NIC card, switch port, or cabling.
Using mii-tool
One of the original Linux tools for setting the speed and duplex of your NIC card was the mii-tool command. It is destined to be deprecated and replaced by the newer ethtool command, but many older NICs support only mii-tool so you'll need to be aware of it. Issuing the command without any arguments gives a brief status report, as seen in the next example, with unsupported NICs providing an Operation not supported message. NICs that are not compatible with mii-tool often will still work, but you have to refer to the manufacturer's guides to set the speed and duplex to anything but auto-negotiate.
[root@bigboy tmp]# mii-tool
SIOCGMIIPHY on 'eth0' failed: Operation not supported eth1: 100 Mbit, half duplex, link ok [root@bigboy tmp]#
By using the verbose mode -v switch you can get much more information. In this case, negotiation was OK, with the NIC selecting 100Mbps, full duplex mode (FD):
[root@bigboy tmp]# mii-tool -v
eth1: negotiated 100baseTx-FD, link ok
product info: vendor 00:10:18, model 33 rev 2 basic mode: autonegotiation enabled basic status: autonegotiation complete, link ok capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
[root@bigboy tmp]#
Setting Your NIC's Speed Parameters with mii-tool
You can set your NIC to force itself to a particular speed and duplex by using the -F switch with any of the following options: 100baseTx-FD, 100baseTx-HD, 10baseT-FD, or 10baseT-HD. Remember that you could lose all network connectivity to your server if you force your NIC to a particular speed/duplex that doesn't match that of your switch:
[root@bigboy tmp]# mii-tool -F 100baseTx-FD eth0
Unfortunately there is no way to set this on reboot permanently except by placing it the command in the /etc/rc.local file to let it be run at the very end of the booting process or by creating your own startup script if you need it set earlier. Creating your own startup scripts is covered in Chapter 7, "The Linux Boot Process".
Using ethtool
The ethtool command is slated to be the replacement for mii-tool in the near future and tends to be supported by newer NIC cards.
The command provides the status of the interface you provide as its argument. Here we see interface eth0 not doing autonegotiation and set to a speed of 100 Mbps, full duplex. A list of supported modes is also provided at the top of the output.
[root@bigboy tmp]# ethtool eth0
Settings for eth0:
Supported ports: [ TP MII ] Supported link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full Supports auto-negotiation: Yes Advertised link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full Advertised auto-negotiation: No Speed: 100Mb/s Duplex: Full Port: MII PHYAD: 1 Transceiver: internal Auto-negotiation: off Supports Wake-on: g Wake-on: g Current message level: 0x00000007 (7) Link detected: yes
[root@bigboy tmp]#
Setting Your NIC's Speed Parameters with ethtool
Unlike mii-tool, ethtool settings can be permanently set as part of the interface's configuration script with the ETHTOOL_OPTS variable. In our next example, the settings will be set to 100 Mbps, full duplex with no chance for auto-negotiation on the next reboot:
#
- File: /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0 IPADDR=192.168.1.100 NETMASK=255.255.255.0 BOOTPROTO=static ONBOOT=yes ETHTOOL_OPTS="speed 100 duplex full autoneg off"
You can test the application of these parameters by shutting down the interface and activating it again with the ifup and ifdown commands. These settings can also be changed from the command line using the -s switch followed by the interface name and its desired configuration parameters.
[root@bigboy tmp]# ethtool -s eth1 speed 100 duplex full autoneg off
[root@bigboy tmp]#
The Linux man pages give more details on other ethtool options, but you can get a quick guide by just entering the ethtool command alone, which provides a quicker summary.
[root@bigboy tmp]# ethtool
... ...
ethtool -s DEVNAME \ [ speed 10|100|1000 ] \ [ duplex half|full ] \ [ port tp|aui|bnc|mii|fibre ] \
... ... [root@bigboy tmp]#
A Note About Duplex Settings
By default, Linux NICs negotiate their speed and duplex settings with the switch. This is done by exchanging electronic signals called Fast Link Pulses (FLP). When the speed and duplex are forced to a particular setting the FLPs are not sent. When a NIC is in auto-negotiation mode and detects a healthy, viable link but receives no FLPs, it errs on the side of caution and sets its duplex to half-duplex and sometimes it will also set its speed to the lowest configurable value. It is therefore possible to force a switch port to 100 Mbps full duplex, but have the auto-negotiating server NIC set itself to 100Mbps half-duplex which will result in errors. The same is true for the switch if the switch port is set to auto-negotiate and server NIC is set to 100 Mbps full duplex. It is best to either force both the switch port and server NIC to either auto-negotiate or the same forced speed and duplex values.
How to Convert Your Linux Server into a Simple Router
Router/firewall appliances that provide basic Internet connectivity for a small office or home network are becoming more affordable every day, but when budgets are tight you might seriously want to consider modifying an existing Linux server to do the job.
Details on how to configure Linux firewall security are covered in Chapter 14, "Linux Firewalls Using iptables", but you need to understand how to activate routing through the firewall before it can become a functioning networking device.
Configuring IP Forwarding
For your Linux server to become a router, you have to enable packet forwarding. In simple terms packet forwarding enables packets to flow through the Linux box from one network to another. The Linux kernel configuration parameter to activate this is named net.ipv4.ip_forward and can be found in the file /etc/sysctl.conf. Remove the "#" from the line related to packet forwarding.
Before:
- Disables packet forwarding
net.ipv4.ip_forward=0
After:
- Enables packet forwarding
net.ipv4.ip_forward=1
This enables packet forwarding only when you reboot at which time Linux will create a file in one of the subdirectories of the special RAM memory-based /proc filesystem. To activate the feature immediately you have to force Linux to read the /etc/sysctl.conf file with the sysctl command using the -p switch. Here is how it's done:
[root@bigboy tmp] sysctl -p
sysctl -p net.ipv4.ip_forward = 1 net.ipv4.conf.default.rp_filter = 1 kernel.sysrq = 0 kernel.core_uses_pid = 1 [root@bigboy tmp]#
Please refer to Appendix I for more information on adjusting kernel parameters.
Configuring Proxy ARP
If a server needs to send a packet to another device on the same network, it sends out an ARP request to the network asking for the MAC address of the other device.
If the same server needs to send a packet to another device on a remote network the process is different. The server first takes a look at its routing table to find out the IP address of the best router on its network that will be able to relay the packet to the destination. The server then sends an ARP request for the MAC address that matches the router's IP address. It then sends the packet to the router using the router's MAC address and a destination IP address of the remote server.
If there is no suitable router on its network, the server will then send out an ARP request for the MAC address of the remote server. Some routers can be configured to answer these types of ARP requests for remote networks. This feature is called proxy ARP. There are some disadvantages with this. One of the most common problems occurs if two routers are on the network configured for proxy ARP. In this scenario there is the possibility that either one will answer the local server's ARP request for the MAC address of the remote server. If one of the routers has an incorrect routing table entry for the remote network, then there is the risk that traffic to the remote server will occasionally get lost. In other words you can lose routing control.
Note: It is for this and other reasons that it is generally not a good idea to configure proxy ARP on a router. It is also good to always configure a default gateway on your server and use separate routing entries via other routers for all networks your default gateway may not know about.
Some types of bridging mode firewalls need to have proxy ARP enabled to operate properly. These devices are typically inserted as part of a daisy chain connecting multiple network switches together on the same LAN while protecting one section of a LAN from traffic originating on another section. The firewall typically isn't configured with an IP address on the LAN and appears to be an intelligent cable capable of selectively blocking packets.
If you need to enable proxy ARP on a Linux server the /proc filesystem comes into play again. Proxy ARP is handled by files in the /proc/sys/net/ipv4/conf/ directory. This directory then has subdirectories corresponding to each functioning NIC card on your server. Each subdirectory then has a file called proxy_arp. If the value within this file is 0, then proxy ARP on the interface is disabled; if the value is 1 then it is enabled.
You can use the /etc/sysctl.conf file mentioned in Appendix II to activate or disable proxy ARP. The next example activates proxy ARP, first for all interfaces and then for interfaces eth0 and wlan0.
#
- File: /etc/sysctl.conf
- Enables Proxy ARP on all interfaces
net/ipv4/conf/all/proxy_arp = 1
- Enables Proxy ARP on interfaces eth1 and wlan0
net/ipv4/conf/eth1/proxy_arp = 1 net/ipv4/conf/wlan0/proxy_arp = 1
You can then activate these settings with the sysctl command.
[root@bigboy tmp] sysctl -p
Configuring Your /etc/hosts File
The /etc/hosts file is just a list of IP addresses and their corresponding server names. Your server will typically check this file before referencing DNS. If the name is found with a corresponding IP address then DNS won't be queried at all. Unfortunately, if the IP address for that host changes, you also have to also update the file. This may not be much of a concern for a single server, but can become laborious if it has to be done companywide. For ease of management, it is often easiest to limit entries in this file to just the loopback interface and also the server's own hostname, and use a centralized DNS server to handle most of the rest. Sometimes you might not be the one managing the DNS server, and in such cases it may be easier to add a quick /etc/hosts file entry till the centralized change can be made.
192.168.1.101 smallfry
In the example above server smallfry has an IP address of 192.168.1.101. You can access 192.168.1.101 using the ping, telnet or any other network aware program by referring to it as smallfry. Here is an example using the ping command to see whether smallfry is alive and well on the network:
[root@bigboy tmp]# ping smallfry
PING zero (192.168.1.101) 56(84) bytes of data. 64 bytes from smallfry (192.168.1.101): icmp_seq=0 ttl=64 time=0.197 ms 64 bytes from smallfry (192.168.1.101): icmp_seq=1 ttl=64 time=0.047 ms
--- smallfry ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 2017ms
rtt min/avg/max/mdev = 0.034/0.092/0.197/0.074 ms, pipe 2
[root@bigboy tmp]#
You can also add aliases to the end of the line which enable you to refer to the server using other names. Here we have set it up so that smallfry can also be accessed using the names tiny and littleguy.
192.168.1.101 smallfry tiny littleguy
You should never have an IP address more than once in this file because Linux will use only the values in the first entry it finds.
192.168.1.101 smallfry # (Wrong)
192.168.1.101 tiny # (Wrong) 192.168.1.101 littleguy # (Wrong)
The loopback Interface's localhost Entry
Usually the first entry in /etc/hosts defines the IP address of the server's virtual loopback interface. This is usually mapped to the name localhost.localdomain (the universal name used when a server refers to itself) and localhost (the shortened alias name). By default, Fedora inserts the hostname of the server between the 127.0.0.1 and the localhost entries like this:
127.0.0.1 bigboy localhost.localdomain localhost
When the server is connected to the Internet this first entry after the 127.0.0.1 needs to be the fully qualified domain name (FQDN) of the server. For example, bigboy.mysite.com, like this:
127.0.0.1 bigboy.my-site.com localhost.localdomain localhost
Some programs such as Sendmail are very sensitive to this and if they detect what they feel is an incorrect FQDN they will default to using the name localhost.localdomain when communicating with another server on the network. This can cause confusion, as the other server also feels it is localhost.localdomain.
Note: You must always have a localhost and localhost.localdomain entry mapping to 127.0.0.1 for Linux to work properly and securely.
Debian / Ubuntu Network Configuration
Many of the core Fedora / Redhat commands and configuration files covered in this chapter can be used in Debian based operating systems, but there are some key differences.
The /etc/network/interfaces File
The main network configuration file is the /etc/network/interfaces file in which all the network interface parameters are defined. The file is divided into stanzas:
The auto Stanza
The auto stanza defines the interfaces that should be automatically initialized when the system boots up.
The mapping Stanza
This stanza maps configuration parameters for an interface depending on the output of a script. For example, on booting the script could prompt you as to whether your laptop Linux system is at home or work with the mapping statement using the answer to configure the appropriate IP address.
By default the much simpler hotplug system is used which assumes that the interfaces will have only one purpose. Typical hotplug configurations simply assign each physical interface with a matching logical interface name (nick name).
mapping hotplug script grep map eth0 eth0 map eth1
In this case interface eth0 is specifically given the logical name eth0, while the logical name for eth1 is implied to be the same.
The iface Stanza
The iface stanza defines the characteristics of a logical interface. Typically the first line of these stanzas starts with the word iface, followed by the logical name of the interface, the protocol used, and finally the type of addressing scheme to be used, such as DHCP or static. Protocol keywords include inet for regular TCP/IP, inet6 for IPv6, ipx for the older IPX protocol used by Novell, and loopback for loopback addresses.
Subsequent lines in the stanza define protocol characteristics such as addresses, subnet masks, and default gateways. In this example, interface eth1 is given the IP address 216.10.119.240/27 while interface eth0 gets its IP address using DHCP.
# The primary network interface
auto eth1 iface eth1 inet static
address 216.10.119.240 netmask 255.255.255.224 network 216.10.119.224 broadcast 216.10.119.255 gateway 216.10.119.241 dns-nameservers 216.10.119.241
- The secondary network interface
auto eth0 iface eth0 inet dhcp
Note: When static IP addresses are used, a default gateway usually needs to be defined. Remember to place the gateway statement in the correct stanza with the appropriate router IP address.
Creating Interface Aliases
IP aliases can be easily created in the /etc/network/interfaces file once the main interface has already been defined. A modified duplicate of the main interfaces' iface stanza is required. A colon followed by the sub interface number needs to be added to the first line, and only the subnet mask and the new IP address needs to follow as can be seen in this example for interface eth1:1 with the IP address 216.10.119.239.
auto eth1:1
iface eth1:1 inet static
address 216.10.119.239 netmask 255.255.255.224
Adding Permanent Static Routes
The up option in the appropriate iface stanza of the /etc/network/interfaces file allows you to selectively run commands once the specified interface becomes activated with the ifup command. This makes it useful when adding permanent static routes.
In this example, a route to the 10.0.0.0/8 network via router address 216.10.119.225 has been added. Remember, the up option and the command must reside on the same line of the stanza.
# The primary network interface
auto eth1 iface eth1 inet static
... ... ... up route add -net 10.0.0.0 netmask 255.0.0.0 gw 216.10.119.225 eth1
A complete /etc/network/interfaces file
We can now construct a complete file based on the previous examples we discussed. Just like in Fedora, interfaces can be activated with the ifup and ifdown commands.
#
- Debian / Ubuntu
- File: /etc/network/interfaces
- The loopback network interface
auto lo iface lo inet loopback
- This is a list of hotpluggable network interfaces.
- They will be activated automatically by the hotplug subsystem.
mapping hotplug
script grep map eth0 eth0 map eth1 eth1
- The primary network interface
auto eth1 iface eth1 inet static
address 216.10.119.240 netmask 255.255.255.224 network 216.10.119.224 broadcast 216.10.119.255 gateway 216.10.119.241 # dns-* options are implemented by the resolvconf package, if installed dns-nameservers 216.10.119.241 wireless-key 98d126d5ac wireless-essid schaaffe
up route add -net 10.0.0.0 netmask 255.0.0.0 gw 216.10.119.225 eth1
auto eth1:1 iface eth1:1 inet static
address 216.10.119.239 netmask 255.255.255.224
- The secondary network interface
auto eth0 iface eth0 inet dhcp
For more information on the /etc/network/interfaces file just issue the command man interfaces from the command line.
结论
你可以想象，配置Linux的网络只是提供互联网接入到您的服务器第一步。实际操作中，总是存在一些事情可能出错，可完全脱离你的控制。好的系统管理员了解能够找出这些问题可能原因的所需工具，使他们能够找到相应类型的帮助以解决它。未来两章向您展示如何在事情出错时自信地调试您的网络和Linux服务器应用。对你的公司和事业来说，你查明和纠正这些问题的能力将会被证明是非常宝贵的。
